Electrostatic means for liquid dispersion in minute droplets are used in a variety of technological applications. In some hybrid systems, electric fields are used just for charging purposes, and atomization is achieved by other means, e.g. pneumatic, ultrasonic, etc. In other systems, the dispersion of the liquid is driven primarily by electric forces, so that atomization and gas flow processes are relatively uncoupled. Those in the latter category are referred to as electrosprays. A background discussion of electrospray can be found in U.S. Pat. No. 5,523,566 to Fuerstenau et al., the disclosure of which is incorporated herein by reference.
Within the electrospray class of atomizers is a particular type characterized by the additional feature of a tight control of the size distribution of the resulting aerosol. Such a system can be implemented by feeding a liquid with sufficient electric conductivity through a conductive tube maintained at several kilovolts relative to a reference electrode positioned a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up farther downstream into a spray of fine, charged droplets. In view of the morphology of the liquid meniscus, this regime is labeled as the cone-jet mode.
The cone-jet mode offers not only the appealing feature of droplet monodispersion, but it can produce droplets/particles over a wide size range, from molecular dimensions to hundreds of microns, depending on liquid flow rate, applied voltage and liquid electric conductivity. Especially in the nanometric range, the capability of producing monodisperse particles with relative ease is unmatched by any other aerosol generation scheme. Just as important is the fact that these particles are generated from capillaries with a relatively large bore (e.g., 100 micrometers is a typical figure), which are therefore unlikely to clog.
Another distinctive advantage that the electrostatic approach offers over alternative atomization techniques relates to the presence of a net charge on the surface of the generated droplets. This charge provides a "handle" to guide the particles and to collect them on suitable targets for a variety of purposes. The Coulombic repulsion among droplets prevents any agglomeration by causing droplet self-dispersion.
As the charged droplets evaporate, the electric charge may cause them to subdivide, which eventually creates arbitrarily small drops, residue particles and even ions. If necessary, charge on the droplets can be neutralized after liquid dispersion, using a corona discharge of opposite polarity, a flame, or a radioactive source.
The most important liquid physical properties that govern the electrospray performance are the electric conductivity and the surface tension of the liquid. The electric conductivity defines the range of liquid flow rate over which a) the cone-jet mode can be maintained in a stable manner and b) a uniform droplet size can be produced. Since there is a one-to-one correspondence between liquid flow rate and droplet size, the electric conductivity also governs the range of sizes that can be uniformly produced for a certain liquid. Surface tension intervenes because the voltage required to establish the cone-jet mode is proportional to the square root of this property.
Because of the large surface tension of some liquids, the establishment of stable sprays in air is generally prevented by the occurrence of electric breakdown in the gaseous environment surrounding the spray. The ensuing unsteady corona discharge has destabilizing consequences on the spray behavior and typically prevents operation in the cone-jet mode. For liquids of moderate surface tension, such as aqueous solutions, when air is replaced with CO.sub.2, a gas with higher breakdown threshold, operation in the cone-jet mode is possible, as first demonstrated over 80 years ago by the pioneering work of Zeleny. For example, by this approach, droplets of nearly monodisperse size distributions in the 2-8 micrometer diameter range were produced in the case of pure water and hypotonic saline solutions (0.005% NaCl) at flow rates ranging from 7 to 20 microliters/min. Liquid metals, on the other hand, having very large surface tensions have been electrosprayed only in vacuum or inside dielectric liquids.
Electrospray has found numerous applications in a variety of fields. The leading one is Electrospray Ionization (ESI) as a means to introduce, in the gas phase, ions pre-existing in solution, including multiply charged macromolecules. Such ions can be analyzed in a mass spectrometer, through so-called Electrospray Mass Spectrometry (ESMS). What ESMS did to launch the rapidly growing popularity of this technique was to show that large dissolved macromolecules appeared in the gas as multiply charged ions, with sufficient charge to be mass analyzed in instruments with a relatively small mass range, even when the mass of these ions exceeded several millions. Well over 90% of the literature on electrosprays is now devoted to ESMS of large polymers.
The ability of this technique to convert dissolved entities into their charged gas phase counterparts is equally applicable to small inorganic clusters, branched polymers, particles, viruses, or other liquid contaminants of interest.
Other applications have been demonstrated in the laboratory but have yet to find an industrial follow-up, partly because of the relatively small flow rates that the electrospray can deliver. For instance, an electrospray has been shown to be capable of producing droplets of saline aqueous solutions in a size range desirable for drug inhalation therapy, targeted to specific areas of the respiratory tract.
The production of metal clusters and ions by conventional electrosprays typically entails the melting of the metal and the establishment of metal cones under the action of electric fields. High vacuum is usually required because of the high surface tension of the metal. In this way, in the late sixties, the production of Cs.sup.+ monomers, dimers and trimers, as well as alkali ions beams was demonstrated. The approach was found to be well-suited for liquid metals of low work functions, whereas higher work functions metals may produce charged droplets and ion clusters at the same time.
Scaling laws for the diameter of electrosprayed droplets indicate that it is possible, from highly conducting solutions, to produce drops with diameters of several tens of nanometers. Even finer particles with controlled and narrow distribution of sizes can be produced by dissolving a nonvolatile solute into an electrosprayable solvent. After electrospray dispersion, the solvent evaporates leaving behind nanometric residues, that can be used for specific applications.
It is an object of this invention to provide an improved electrospray apparatus and method which enables production of highly uniform size particles.
It is another object of this invention to provide an improved electrospray apparatus and method which, for a given liquid solution, enables production of particles of smaller sizes and at lower liquid flow rates than particles is produced by prior art electrosprays.